Methods and systems for detecting atrial contraction timing fiducials during ventricular filling from a ventricularly implanted leadless cardiac pacemaker

ABSTRACT

A ventricularly implantable medical device that includes a sensing module that is configured to detect an artifact during ventricular filling and to identify an atrial event based at least on part on the detected artifact. Control circuitry of the implantable medical device is configured to deliver a ventricular pacing therapy to a patient&#39;s heart, wherein the ventricular pacing therapy is time dependent, at least in part, on the identified atrial event.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/593,642 filed on Dec. 1, 2017, the disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally relates to implantable medical devices,and more particularly, to systems that use a leadless cardiac pacemakerfor monitoring, pacing and/or defibrillating a patient's heart

BACKGROUND

Implantable medical devices are commonly used today to monitor a patientand/or deliver therapy to a patient. For example, and in some instances,pacing devices are used to treat patients suffering from various heartconditions that may result in a reduced ability of the heart to deliversufficient amounts of blood to a patient's body. Such heart conditionsmay lead to slow, rapid, irregular, and/or inefficient heartcontractions. To help alleviate some of these conditions, variousmedical devices (e.g., pacemakers, defibrillators, etc.) can beimplanted in a patient's body. Such devices may monitor and in somecases provide electrical stimulation (e.g. pacing, defibrillation, etc.)to the heart to help the heart operate in a more normal, efficientand/or safe manner. In some cases, it may be beneficial to detectcardiac events occurring in multiple chambers of the heart. In somecases, this may be used to enhance the effectiveness of the cardiacpacing therapy and/or may allow different types of cardiac pacingtherapy to be delivered.

SUMMARY

This disclosure generally relates to implantable medical devices, andmore particularly, to systems that use a leadless cardiac pacemaker formonitoring, pacing and/or defibrillating a patient's heart.

In a first example, a leadless cardiac pacemaker (LCP) may be configuredto sense cardiac activity and to deliver pacing therapy to a ventricleof a patient's heart. The LCP may comprise a housing having a proximalend and a distal end, a first electrode secured relative to the housingand exposed to the environment outside of the housing, a secondelectrode secured relative to the housing and exposed to the environmentoutside of the housing, a sensing module disposed within the housing,the sensing module configured to detect an artifact during ventricularfilling and identify an atrial event based, at least in part, on thedetected artifact, and circuitry in the housing operatively coupled tothe first electrode and the second electrode, and also operativelycoupled to the sensing module, the circuitry configured to deliver aventricular pacing therapy to the patient's heart via the firstelectrode and the second electrode, wherein the ventricular pacingtherapy is time dependent, at least in part, on the identified atrialevent.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to detect an atrialartifact during passive ventricular filling.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to sense at least one of apressure, impedance, strain, sound, rotation, acceleration, voltage, andflow.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may comprise a filter for filtering a signalfor the at least one of pressure, impedance, strain, sound, rotation,acceleration, voltage, and flow.

Alternatively or additionally to any of the examples above, in anotherexample, the filter may include a first filter for passing a firstfrequency band, a second filter for passing a second frequency band anda third filter for passing a third frequency band.

Alternatively or additionally to any of the examples above, in anotherexample, the artifact may comprise at least one of a p-wave, S2 heartsound, S3 heart sound, ventricular volume, ventricular wall dimension,ventricular blood movement, ventricular wall movement, tricuspid valveposition, mitral valve position, and akinetic ventricular pressure.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to detect an atrialartifact during active ventricular filling.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to sense at least one ofan impedance, strain, sound, rotation, or flow.

Alternatively or additionally to any of the examples above, in anotherexample, the artifact may comprise at least one of a p-wave, a-wave,e-wave, S1 heart sound, S4 heart sound, ventricular volume, ventricularwall dimension, cardiac tissue vibration, ventricular blood movement,ventricular wall movement, tricuspid valve position, and mitral valveposition.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may comprise a bandpass filter of about 1Hertz (Hz) to about 5 Hz.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may comprise a bandpass filter of about 15Hertz (Hz) to about 30 Hz.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be further configured to detect at leastone of posture, activity, or respiration.

Alternatively or additionally to any of the examples above, in anotherexample, the circuitry may be further configured to determine anintrinsic interval based, at least in part, on the atrial event.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to calculate an ensembleaverage timing over multiple heart beats for the detected artifact.

Alternatively or additionally to any of the examples above, in anotherexample, the LCP may be configured to be implantable in a ventricle ofthe patient's heart.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to detect an atrialartifact by detecting a transition between passive and activeventricular filling.

In another example, a leadless cardiac pacemaker (LCP) may be configuredto sense cardiac activity and to deliver pacing therapy to a ventricleof a patient's heart. The LCP may comprise a housing having a proximalend and a distal end, a first electrode secured relative to the housingand exposed to the environment outside of the housing, a secondelectrode secured relative to the housing and exposed to the environmentoutside of the housing, a sensing module disposed within the housing,the sensing module configured to detect an artifact during ventricularfilling and identify an atrial event based, at least in part, on thedetected artifact, and circuitry in the housing operatively coupled tothe first electrode and the second electrode, and also operativelycoupled to the sensing module, the circuitry configured to deliver aventricular pacing therapy to the patient's heart via the firstelectrode and the second electrode, wherein the ventricular pacingtherapy is time dependent, at least in part, on the identified atrialevent.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to detect an atrialartifact during passive ventricular filling.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to sense at least one of apressure, impedance, strain, sound, rotation, acceleration, voltage, andflow.

Alternatively or additionally to any of the examples above, in anotherexample, the artifact may comprise at least one of a p-wave, S2 heartsound, S3 heart sound, ventricular volume, ventricular wall dimension,ventricular blood movement, ventricular wall movement, tricuspid valveposition, mitral valve position, and akinetic ventricular pressure.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to detect an atrialartifact during active ventricular filling.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to sense at least one ofan impedance, strain, sound, rotation, or flow.

Alternatively or additionally to any of the examples above, in anotherexample, the artifact may comprise at least one of p-wave, a-wave,e-wave, S1 heart sound, S4 heart sound, ventricular volume, ventricularwall dimension, cardiac tissue vibration, ventricular blood movement,ventricular wall movement, tricuspid valve position, and mitral valveposition.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be further configured to detect at leastone of posture, activity, or respiration.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to detect an atrialartifact by detecting a transition between passive and activeventricular filling.

Alternatively or additionally to any of the examples above, in anotherexample, the circuitry may be further configured to determine anintrinsic interval based, at least in part, on the atrial event.

In another example, an implantable medical device may be configured tosense cardiac activity and to deliver pacing therapy to a patient'sheart. The implantable medical device may comprise a housing, a firstelectrode secured relative to the housing and exposed to the environmentoutside of the housing, a second electrode secured relative to thehousing and exposed to the environment outside of the housing, and asensing module disposed within the housing. The sensing module may beconfigured to sense during passive ventricular filling of the patient'sheart at least one of a pressure, impedance, strain, sound, rotation,acceleration, voltage, and flow, detect an artifact, and identify anatrial event based, at least in part, on the detected artifact. Theimplantable medical device may further comprise circuitry in the housingoperatively coupled to the first electrode, the second electrode, andthe sensing module, the circuitry configured to deliver a ventricularpacing pulse to the patient's heart via the first electrode and thesecond electrode, wherein a timing of the ventricular pacing pulse isdependent, at least in part, on the timing of the identified atrialevent.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may comprise a filter for filtering a signalfor the at least one of pressure, impedance, strain, sound, rotation,acceleration, voltage, and flow.

Alternatively or additionally to any of the examples above, in anotherexample, the filter may include a first filter for passing a firstfrequency band, a second filter for passing a second frequency band anda third filter for passing a third frequency band.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may be configured to calculate an ensembleaverage timing over multiple heart beats for the detected artifact.

Alternatively or additionally to any of the examples above, in anotherexample, the detected artifact may comprise at least one of a p-wave, S2heart sound, S3 heart sound, ventricular volume, ventricular walldimension, ventricular blood movement, ventricular wall movement,tricuspid valve position, mitral valve position, and akineticventricular pressure.

Alternatively or additionally to any of the examples above, in anotherexample, the implantable medical device may be configured to beimplantable in a ventricle of the patient's heart and the sensing moduleidentifies an atrial event based, at least in part, on the detectedartifact.

In another example, an implantable medical device may be configured tosense cardiac activity and to deliver pacing therapy to a patient'sheart. The implantable medical device may comprise a housing, a firstelectrode secured relative to the housing and exposed to the environmentoutside of the housing, a second electrode secured relative to thehousing and exposed to the environment outside of the housing, and asensing module disposed within the housing. The sensing module may beconfigured to sense during active ventricular filling of the patient'sheart at least one of a pressure, impedance, strain, sound, rotation,acceleration, voltage, and flow, detect an artifact, and identify anatrial event based, at least in part, on the detected artifact. Theimplantable medical device may further comprise circuitry in the housingoperatively coupled to the first electrode, the second electrode, andthe sensing module, the circuitry is configured to deliver a ventricularpacing therapy to the patient's heart via the first electrode and thesecond electrode, wherein the ventricular pacing therapy is timedependent, at least in part, on the identified atrial event.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may comprise a bandpass filter of about 1Hertz (Hz) to about 5 Hz.

Alternatively or additionally to any of the examples above, in anotherexample, the sensing module may comprise a bandpass filter of about 15Hertz (Hz) to about 30 Hz.

Alternatively or additionally to any of the examples above, in anotherexample, the artifact may comprise at least one of a p-wave, a-wave,e-wave, S1 heart sound, S4 heart sound, ventricular volume, ventricularwall dimension, cardiac tissue vibration, ventricular blood movement,ventricular wall movement, tricuspid valve position, and mitral valveposition.

Alternatively or additionally to any of the examples above, in anotherexample, the implantable medical device may be configured to beimplantable in a ventricle of the patient's heart and the sensing moduleidentifies an atrial event based, at least in part, on the detectedartifact.

The above summary is not intended to describe each embodiment or everyimplementation of the present disclosure. Advantages and attainments,together with a more complete understanding of the disclosure, willbecome apparent and appreciated by referring to the followingdescription and claims taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing description of various illustrative embodiments in connectionwith the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of an illustrative leadless cardiacpacemaker (LCP) according to one example of the present disclosure;

FIG. 2 a schematic block diagram of another medical device (MID), whichmay be used in conjunction with an LCP 100 (FIG. 1) in order to detectand/or treat cardiac arrhythmias and other heart conditions;

FIG. 3 is a schematic diagram of an exemplary medical system thatincludes an LCP and another medical device, in accordance with yetanother example of the present disclosure;

FIG. 4 is a graphical representation of an illustrativeelectrocardiogram (ECG) showing a temporal relationship betweenelectrical signals of the heart and mechanical indications ofcontraction of the heart;

FIG. 5 is a graph showing example pressures and volumes within the heartover time;

FIG. 6 is an illustrative table of various artifacts occurring duringthe cardiac cycle and different ways to detect them;

FIG. 7 is an illustrative table of various artifacts occurring duringthe cardiac cycle and during which cardiac phase each occur;

FIG. 8 is a side view of an illustrative LCP;

FIG. 9A is a partial cross-sectional plan view of an example LCPimplanted within a heart during ventricular filling;

FIG. 9B is a partial cross-sectional plan view of an example LCPimplanted within a heart during ventricular contraction;

FIG. 10 is a flow diagram showing an illustrative method of detectingatrial activity from an LCP implanted in a ventricle of the heart andgenerating and delivering a ventricular pacing pulse using the same;

FIG. 11 is a schematic diagram of an illustrative signal averagingmethod that can be used by an LCP implanted in the ventricle to helpidentify atrial timing fiducials;

FIG. 12 shows a portion of an illustrative ventricle pressure signal;

FIG. 13 shows a graph of illustrative cardiac signals including heartsounds, right ventricular pressure, and an electrocardiogram, along withvarious intervals between detectable characteristics of such signals;

FIG. 14 shows a graph of illustrative cardiac signals including heartsounds, right ventricular pressure, and an electrocardiogram, along withvarious timing delays (AV intervals) from detectable characteristics ofsuch signals to a desired ventricle pacing pulse;

FIG. 15 is an illustrative method for determining when a medical deviceshould utilize reversion;

FIG. 16 illustrates a comparison of pacing intervals on anelectrocardiogram when the device is operating in a normal VDD trackingmode and pacing intervals on an electrocardiogram when the device isoperating in a VDD pseudo tracking mode; and

FIG. 17 is a graphic representation of higher order derivatives that canbe used by an LCP implanted in the ventricle to help identify atrialtiming fiducials.

While the disclosure is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit aspects of thedisclosure to the particular illustrative embodiments described. On thecontrary, the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure.

DESCRIPTION

The following description should be read with reference to the drawingsin which similar elements in different drawings are numbered the same.The description and the drawings, which are not necessarily to scale,depict illustrative embodiments and are not intended to limit the scopeof the disclosure. While the present disclosure is applicable to anysuitable implantable medical device (IMD), the description below usespacemakers and more particularly leadless cardiac pacemakers (LCP) asparticular examples.

All numbers are herein assumed to be modified by the term “about”,unless the content clearly dictates otherwise. The recitation ofnumerical ranges by endpoints includes all numbers subsumed within thatrange (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5).

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include the plural referents unless thecontent clearly dictates otherwise. As used in this specification andthe appended claims, the term “or” is generally employed in its senseincluding “and/or” unless the content clearly dictates otherwise.

It is noted that references in the specification to “an embodiment”,“some embodiments”, “other embodiments”, etc., indicate that theembodiment described may include a particular feature, structure, orcharacteristic, but every embodiment may not necessarily include theparticular feature, structure, or characteristic. Moreover, such phrasesare not necessarily referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with an embodiment, it is contemplated that the feature,structure, or characteristic may be applied to other embodiments whetheror not explicitly described unless clearly stated to the contrary.

A normal, healthy heart induces contraction by conducting intrinsicallygenerated electrical signals throughout the heart. These intrinsicsignals cause the muscle cells or tissue of the heart to contract in acoordinated manner. These contractions force blood out of and into theheart, providing circulation of the blood throughout the rest of thebody. Many patients suffer from cardiac conditions that affect theefficient operation of their hearts. For example, some hearts developdiseased tissue that no longer generate or efficiently conduct intrinsicelectrical signals. In some examples, diseased cardiac tissue mayconduct electrical signals at differing rates, thereby causing anunsynchronized and inefficient contraction of the heart. In otherexamples, a heart may generate intrinsic signals at such a low rate thatthe heart rate becomes dangerously low. In still other examples, a heartmay generate electrical signals at an unusually high rate, evenresulting in cardiac fibrillation. In some cases such an abnormality candevelop into a fibrillation state, where the contraction of thepatient's heart chambers are almost completely de-synchronized and theheart pumps very little to no blood. Implantable medical devices, whichmay be configured to determine occurrences of such cardiac abnormalitiesor arrhythmias and deliver one or more types of electrical stimulationtherapy to patient's hearts, may help to terminate or alleviate theseand other cardiac conditions.

It is contemplated that atrial events or artifacts indicative of anatrial event may be used by a device implanted in the right (or left)ventricle to time a pacing pulse for the ventricle in support oftreating bradycardia events. In some cases, the timing of the ventriclepacing pulse may be adjusted to maximize the amount of blood enteringthe right ventricle through passive filling. In some instances, this mayinclude adjusting an AV delay relative to an atrial fiducial (e.g.,atrial kick). In some cases, a measured pressure change (or other atrialfiducial) over time may be used to support management of a CRT cardiactherapy (e.g. if placed in the left ventricle), patient health statusmonitoring and/or any other suitable goal. It is contemplated thatmeasuring events in one of or both of the ventricle and atrium using asingle leadless cardiac pacemaker may replicate a dual chamber systemusing only a single device. For example, such a system may enable adevice to be positioned in a ventricle and capable of sensing intrinsicventricular and atrial events and pacing the ventricle when appropriate(e.g., a VDD pacemaker).

FIG. 1 depicts an illustrative leadless cardiac pacemaker (LCP) that maybe implanted into a patient to provide bradycardia therapy, cardiacresynchronization therapy (CRT), anti-tachycardia pacing (ATP) therapy,defibrillation therapy, and/or the like. As can be seen in FIG. 1, theillustrative LCP 100 may be a compact device with all components housedwithin and/or on the LCP housing 120. In the example shown in FIG. 1,the LCP 100 includes a communication module 102, a pulse generatormodule 104, an electrical sensing module 106, a mechanical sensingmodule 108, a processing module 110, a battery 112, and electrodes 114.It is contemplated that the LCP 100 may include more or less modules,depending on the application.

The communication module 102 may be configured to communicate withremote devices such as sensors, other devices, and/or the like, that arelocated externally and/or internally to the patient's body. The otherdevices may be device primarily functioning as a medical device (e.g. aLCP programmer, an implanted sensor) or a device primarily functioningas a non-medical device (e.g. a personal computer, tablet computer,smart phone, laptop computer or the like). Irrespective of the locationor primary function, the remote devices (i.e., external to the LCP 100but not necessarily external to the patient's body) may communicate withthe LCP 100 via the communication module 102 to accomplish one or moredesired functions. For example, the LCP 100 may communicate information,such as sensed signals, data, instructions, messages, etc., to a remotemedical device through the communication module 102. The remote medicaldevice may then use the communicated signals, data, instructions and/ormessages to perform various functions, such as determining occurrencesof arrhythmias, delivering electrical stimulation therapy, storingreceived data, analyzing received data, transmitting the received datato an external programmer or server or the like for review by aphysician, and/or performing any other suitable function. The LCP 100may additionally receive information such as signals, data, instructionsand/or messages from the remote medical device through the communicationmodule 102, and the LCP 100 may use the received signals, data,instructions and/or messages to perform various functions, such asdetermining occurrences of arrhythmias, delivering electricalstimulation therapy, storing received data, analyzing received data,and/or performing any other suitable function. The communication module102 may be configured to use one or more methods for communicating withremote devices. For example, the communication module 102 maycommunicate via radiofrequency (RF) signals, inductive coupling, opticalsignals, acoustic signals, conducted communication signals, and/or anyother signals suitable for communication.

In the example shown in FIG. 1, the pulse generator module 104 may beelectrically connected to the electrodes 114. In some examples, the LCP100 may include one or more additional electrodes 114′. In suchexamples, the pulse generator 104 may also be electrically connected tothe additional electrodes 114′. The pulse generator module 104 may beconfigured to generate electrical stimulation signals. For example, thepulse generator module 104 may generate electrical stimulation signalsby using energy stored in a battery 112 within the LCP 100 and deliverthe generated electrical stimulation signals via the electrodes 114and/or 114′. Alternatively, or additionally, the pulse generator 104 mayinclude one or more capacitors, and the pulse generator 104 may chargethe one or more capacitors by drawing energy from the battery 112. Thepulse generator 104 may then use the energy of the one or morecapacitors to deliver the generated electrical stimulation signals viathe electrodes 114 and/or 114′. In at least some examples, the pulsegenerator 104 of the LCP 100 may include switching circuitry toselectively connect one or more of the electrodes 114 and/or 114′ to thepulse generator 104 in order to select which of the electrodes 114/114′(and/or other electrodes) that the pulse generator 104 uses to deliverthe electrical stimulation therapy. The pulse generator module 104 maygenerate electrical stimulation signals with particular features or inparticular sequences in order to provide one or multiple of a number ofdifferent stimulation therapies. For example, the pulse generator module104 may be configured to generate electrical stimulation signals toprovide electrical stimulation therapy to combat bradycardia,tachycardia, cardiac dyssynchrony, bradycardia arrhythmias, tachycardiaarrhythmias, fibrillation arrhythmias, cardiac synchronizationarrhythmias and/or to produce any other suitable electrical stimulationtherapy. Some more common electrical stimulation therapies includebradycardia therapy, anti-tachycardia pacing (ATP) therapy, cardiacresynchronization therapy (CRT), and cardioversion/defibrillationtherapy.

In some examples, the LCP 100 may not include a pulse generator 104 ormay turn off the pulse generator 104. When so provided, the LCP 100 maybe a diagnostic only device. In such examples, the LCP 100 may notdeliver electrical stimulation therapy to a patient. Rather, the LCP 100may collect data about cardiac electrical activity and/or otherphysiological parameters of the patient and communicate such data and/ordeterminations to one or more other medical devices via thecommunication module 102.

In some examples, the LCP 100 may include an electrical sensing module106, and in some cases, a mechanical sensing module 108. The electricalsensing module 106 may be configured to sense the cardiac electricalactivity of the heart. For example, the electrical sensing module 106may be connected to the electrodes 114/114′, and the electrical sensingmodule 106 may be configured to receive cardiac electrical signalsconducted through the electrodes 114/114′. The cardiac electricalsignals may represent local information from the chamber (e.g. nearfield) in which the LCP 100 is implanted. For instance, if the LCP 100is implanted within a ventricle of the heart, cardiac electrical signalssensed by the LCP 100 through the electrodes 114/114′ may representventricular cardiac electrical signals, and possibly some weaker atrialelectrical signals. The electrical sensing module 106 may be configuredto detect voltage, current and/or impedance. An electrogram sensingmodule may be provided as a part of the electrical sensing module.

The mechanical sensing module 108 may include one or more sensors, suchas an accelerometer, a gyroscope, a microphone, a hydrophone, a bloodpressure sensor, a heart sound sensor, a blood-oxygen sensor, atemperature sensor, a flow sensor, a strain sensor, and/or any othersuitable sensors that are configured to measure one or more mechanicaland/or chemical parameters of the patient. In some cases, the mechanicalsensing module 108 may include two or more of a pressure measurementmodule, an acoustic measurement module, an acceleration measurementmodule.

Both the electrical sensing module 106 and the mechanical sensing module108 may be connected to a processing module 110, which may providesignals representative of the sensed mechanical parameters. Althoughdescribed with respect to FIG. 1 as separate sensing modules, in somecases, the electrical sensing module 106 and the mechanical sensingmodule 108 may be combined into a single sensing module, as desired.

The electrodes 114/114′ can be secured relative to the housing 120 butexposed to the tissue and/or blood surrounding the LCP 100. In somecases, the electrodes 114 may be generally disposed on or near eitherend of the LCP 100 and may be in electrical communication with one ormore of the modules 102, 104, 106, 108, and 110. The electrodes 114/114′may be supported by the housing 120, although in some examples, theelectrodes 114/114′ may be secured relative to the housing 120 throughshort connecting wires (e.g. tail) such that one or more of theelectrodes 114/114′ may be spaced from the housing 120. In exampleswhere the LCP 100 includes one or more electrodes 114′, the electrodes114′ may in some cases be disposed on the sides of the housing 120 ofthe LCP 100, which may increase the number of electrodes by which theLCP 100 may sense cardiac electrical activity, deliver electricalstimulation and/or communicate with an external medical device. Theelectrodes 114/114′ can be made up of one or more biocompatibleconductive materials such as various metals or alloys that are known tobe safe for implantation within a human body. In some instances, theelectrodes 114/114′ connected to LCP 100 may have an insulative portionthat electrically isolates the electrodes 114/114′ from adjacentelectrodes, the housing 120, and/or other parts of the LCP 100.

The processing module 110 can be configured to control the operation ofthe LCP 100. For example, the processing module 110 may be configured toreceive electrical signals from the electrical sensing module 106 and/orthe mechanical sensing module 108. Based on the received signals, theprocessing module 110 may determine, for example, a need for pacingtherapy such as bradycardia therapy, cardiac resynchronization therapy(CRT), anti-tachycardia pacing (ATP) therapy, defibrillation therapy,and/or the like. The processing module 110 may control the pulsegenerator module 104 to generate electrical stimulation in accordancewith one or more pacing therapies. The processing module 110 may furtherreceive information from the communication module 102. In some examples,the processing module 110 may use such received information to helpdetermine the need for pacing therapy and/or what type of pacingtherapy. The processing module 110 may additionally control thecommunication module 102 to send/receive information to/from otherdevices.

In some examples, the processing module 110 may include a pre-programmedchip, such as a very-large-scale integration (VLSI) chip and/or anapplication specific integrated circuit (ASIC). In such embodiments, thechip may be pre-programmed with control logic in order to control theoperation of the LCP 100. By using a pre-programmed chip, the processingmodule 110 may use less power than other programmable circuits (e.g.,general purpose programmable microprocessors) while still being able tomaintain basic functionality, thereby potentially increasing the batterylife of the LCP 100. In other examples, the processing module 110 mayinclude a programmable microprocessor. Such a programmablemicroprocessor may allow a user to modify the control logic of the LCP100 even after implantation, thereby allowing for greater flexibility ofthe LCP 100 than when using a pre-programmed ASIC. In some examples, theprocessing module 110 may further include a memory, and the processingmodule 110 may store information on and read information from thememory. In other examples, the LCP 100 may include a separate memory(not shown) that is in communication with the processing module 110,such that the processing module 110 may read and write information toand from the separate memory.

The battery 112 may provide power to the LCP 100 for its operations. Insome examples, the battery 112 may be a non-rechargeable lithium-basedbattery. In other examples, a non-rechargeable battery may be made fromother suitable materials, as desired. Because the LCP 100 is animplantable device, access to the LCP 100 may be limited afterimplantation. Accordingly, it is desirable to have sufficient batterycapacity to deliver therapy over a period of treatment such as days,weeks, months, years or even decades. In some instances, the battery 112may a rechargeable battery, which may help increase the useable lifespanof the LCP 100. In still other examples, the battery 112 may be someother type of power source, as desired.

To implant the LCP 100 inside a patient's body, an operator (e.g., aphysician, clinician, etc.), may fix the LCP 100 to the cardiac tissueof the patient's heart. To facilitate fixation, the LCP 100 may includeone or more anchors 116. The anchor 116 may include any one of a numberof fixation or anchoring mechanisms. For example, the anchor 116 mayinclude one or more pins, staples, threads, screws, helix, tines, and/orthe like. In some examples, although not shown, the anchor 116 mayinclude threads on its external surface that may run along at least apartial length of the anchor 116. The threads may provide frictionbetween the cardiac tissue and the anchor to help fix the anchor 116within the cardiac tissue. In other examples, the anchor 116 may includeother structures such as barbs, spikes, or the like to facilitateengagement with the surrounding cardiac tissue.

FIG. 2 depicts an example of another medical device (MD) 200, which maybe used in conjunction with an LCP 100 (FIG. 1) in order to detectand/or treat cardiac arrhythmias and other heart conditions. In theexample shown, the MD 200 may include a communication module 202, apulse generator module 204, an electrical sensing module 206, amechanical sensing module 208, a processing module 210, and a battery218. Each of these modules may be similar to the modules 102, 104, 106,108, and 110 of the LCP 100. Additionally, the battery 218 may besimilar to the battery 112 of the LCP 100. In some examples, the MD 200may have a larger volume within the housing 220 than LCP 100. In suchexamples, the MD 200 may include a larger battery and/or a largerprocessing module 210 capable of handling more complex operations thanthe processing module 110 of the LCP 100.

While it is contemplated that the MD 200 may be another leadless devicesuch as shown in FIG. 1, in some instances the MD 200 may include leadssuch as leads 212. The leads 212 may include electrical wires thatconduct electrical signals between the electrodes 214 and one or moremodules located within the housing 220. In some cases, the leads 212 maybe connected to and extend away from the housing 220 of the MD 200. Insome examples, the leads 212 are implanted on, within, or adjacent to aheart of a patient. The leads 212 may contain one or more electrodes 214positioned at various locations on the leads 212, and in some cases atvarious distances from the housing 220. Some of the leads 212 may onlyinclude a single electrode 214, while other leads 212 may includemultiple electrodes 214. Generally, the electrodes 214 are positioned onthe leads 212 such that when the leads 212 are implanted within thepatient, one or more of the electrodes 214 are positioned to perform adesired function. In some cases, the one or more of the electrodes 214may be in contact with the patient's cardiac tissue. In some cases, theone or more of the electrodes 214 may be positioned substernally orsubcutaneously and spaced from but adjacent to the patient's heart. Insome cases, the electrodes 214 may conduct intrinsically generatedelectrical signals to the leads 212, e.g., signals representative ofintrinsic cardiac electrical activity. The leads 212 may, in turn,conduct the received electrical signals to one or more of the modules202, 204, 206, and 208 of the MD 200. In some cases, the MD 200 maygenerate electrical stimulation signals, and the leads 212 may conductthe generated electrical stimulation signals to the electrodes 214. Theelectrodes 214 may then conduct the electrical signals and deliver thesignals to the patient's heart (either directly or indirectly).

The mechanical sensing module 208, as with the mechanical sensing module108, may contain or be electrically connected to one or more sensors,such as microphones, hydrophones, accelerometers, gyroscopes, bloodpressure sensors, heart sound sensors, blood-oxygen sensors, acousticsensors, ultrasonic sensors, strain sensors, and/or other sensors whichare configured to measure one or more mechanical/chemical parameters ofthe heart and/or patient. In some examples, one or more of the sensorsmay be located on the leads 212, but this is not required. In someexamples, one or more of the sensors may be located in the housing 220.

While not required, in some examples, the MD 200 may be an implantablemedical device. In such examples, the housing 220 of the MD 200 may beimplanted in, for example, a transthoracic region of the patient. Thehousing 220 may generally include any of a number of known materialsthat are safe for implantation in a human body and may, when implanted,hermetically seal the various components of the MD 200 from fluids andtissues of the patient's body.

In some cases, the MD 200 may be an implantable cardiac pacemaker (ICP).In this example, the MD 200 may have one or more leads, for exampleleads 212, which are implanted on or within the patient's heart. The oneor more leads 212 may include one or more electrodes 214 that are incontact with cardiac tissue and/or blood of the patient's heart. The MD200 may be configured to sense intrinsically generated cardiacelectrical signals and determine, for example, one or more cardiacarrhythmias based on analysis of the sensed signals. The MD 200 may beconfigured to deliver CRT, ATP therapy, bradycardia therapy, and/orother therapy types via the leads 212 implanted within the heart or inconcert with the LCP by commanding the LCP to pace. In some examples,the MD 200 may additionally be configured to provide defibrillationtherapy.

In some instances, the MD 200 may be an implantablecardioverter-defibrillator (ICD). In such examples, the MD 200 mayinclude one or more leads implanted within a patient's heart. The MD 200may also be configured to sense cardiac electrical signals, determineoccurrences of tachyarrhythmias based on the sensed signals, and may beconfigured to deliver defibrillation therapy in response to determiningan occurrence of a tachyarrhythmia. In some instances, the MD 200 may bea subcutaneous implantable cardioverter-defibrillator (S-ICD). Inexamples where the MD 200 is an S-ICD, one of the leads 212 may be asubcutaneously or substernally implanted lead that is spaced from theheart. In at least some examples where the MD 200 is an S-ICD, the MD200 may include only a single lead which is implanted subcutaneously orsubsternally, but this is not required. In some cases, the S-ICD leadmay extend subcutaneously from the S-ICD can, around the sternum and mayterminate adjacent the interior surface of the sternum and spaced fromthe heart.

In some examples, the MD 200 may not be an implantable medical device.Rather, the MD 200 may be a device external to the patient's body, andmay include skin-electrodes that are placed on a patient's body. In suchexamples, the MD 200 may be able to sense surface electrical signals(e.g., cardiac electrical signals that are generated by the heart orelectrical signals generated by a device implanted within a patient'sbody and conducted through the body to the skin). In such examples, theMD 200 may be configured to deliver various types of electricalstimulation therapy, including, for example, defibrillation therapy. TheMD 200 may be further configured to deliver electrical stimulation viathe LCP by commanding the LCP to deliver the therapy.

It is contemplated that one or more LCPs 100 and/or one or more MDs 200may be used in combination as an example medical device system. Thevarious devices 100, 200 may communicate through various communicationpathways including using RF signals, inductive coupling, conductivecoupling optical signals, acoustic signals, or any other signalssuitable for communication. The system may further include and be incommunication with a display. The display may be a personal computer,tablet computer, smart phone, laptop computer, or other display asdesired. In some instances, the display may include input means forreceiving an input from a user. For example, the display may alsoinclude a keyboard, mouse, actuatable (e.g., pushable) buttons, or atouchscreen display. These are just examples. Some illustrative medicaldevice systems are described in commonly assigned Patent Application No.62/547,458, entitled IMPLANTABLE MEDICAL DEVICE WITH PRESSURE SENSOR andfiled on Aug. 18, 2017, which is hereby incorporated by reference.

FIG. 3 shows an example system 250 incorporating an LCP 100 and a MD200. In FIG. 3, an LCP 100 is shown fixed to the interior of the rightventricle of the heart H, and MD 200 including a pulse generator isshown coupled to a lead 212 having one or more electrodes 214 a, 214 b,214 c. In some cases, the MD 200 may be part of a subcutaneousimplantable cardioverter-defibrillator (S-ICD), and the one or moreelectrodes 214 a, 214 b, 214 c may be positioned subcutaneously orsubsternally adjacent the heart. In some cases, the S-ICD lead mayextend subcutaneously from the S-ICD can, around the sternum and one ormore electrodes 214 a, 214 b, 214 c may be positioned adjacent theinterior surface of the sternum but spaced from the heart H. In somecases, the LCP 100 may communicate with the subcutaneous implantablecardioverter-defibrillator (S-ICD).

In some cases, the LCP 100 may be in the left ventricle, right atrium orleft atrium of the heart, as desired. In some cases, more than one LCP100 may be implanted. For example, one LCP may be implanted in the rightventricle and another may be implanted in the right atrium. In anotherexample, one LCP may be implanted in the right ventricle and another maybe implanted in the left ventricle. In yet another example, one LCP maybe implanted in each of the chambers of the heart. Further, the LCP 100may be used without the second MD 200.

The medical device system 250 may also include an external supportdevice, such as external support device 260. The external support device260 can be used to perform functions such as device identification,device programming and/or transfer of real-time and/or stored databetween devices using one or more of the communication techniquesdescribed herein. As one example, communication between the externalsupport device 260 and the MD 200 is performed via a wireless mode (e.g.RF, Bluetooth, inductive communication, etc.), and communication betweenthe MD 200 and the LCP 100 is performed via a conducted mode (e.g.conducted communication). In some examples, communication between theLCP 100 and the external support device 260 is accomplished by sendingcommunication information through the MD 200. However, in otherexamples, communication between the LCP 100 and the external supportdevice 260 may be direct. In some embodiments, the external supportdevice 260 may be provided with or be in communication with a display262. The display 262 may be a personal computer, tablet computer, smartphone, laptop computer, or other display as desired. In some instances,the display 262 may include input means for receiving an input from auser. For example, the display 262 may also include a keyboard, mouse,actuatable buttons, or be a touchscreen display. These are justexamples.

With reference to FIG. 4, it will be appreciated that the heart iscontrolled via electrical signals that pass through the cardiac tissueand that can be detected by implanted devices such as but not limited tothe LCP 100 and/or MD 200 of FIG. 1 or 2. FIG. 4 is a graphicalrepresentation of an illustrative electrocardiogram (ECG) 300 showing atemporal relationship between electrical signals of the heart andmechanical indications 302 (e.g. heart sounds) of contraction of theheart. As can be seen in the illustrative ECG 300, a heartbeat includesa P-wave that indicates atrial depolarization associated with an atrialcontraction to load the ventricles. A QRS complex, including a Q-wave,an R-wave and an S-wave, represents a ventricular depolarizationassociated with the ventricles contracting to pump blood to the body andlungs. A T-wave shows the repolarization of the ventricles inpreparation for a next heart beat. With heart disease, the timing ofthese individual events may be anomalous or abnormal, and the shape,amplitude and/or timing of the various waves can be different from thatshown. It will be appreciated that the ECG 300 may be detected byimplanted devices such as but not limited to the LCP 100 and/or MD 200of FIG. 1 or 2.

The electrical signal 300 typically instructs a portion of the heart tocontract, which then results in a corresponding mechanical contraction.There is a correspondence between a characteristic in the electricalsignal (e.g. ECG 300) and a corresponding mechanical response. Themechanical response is typically delayed because it takes some time forthe heart to respond to the electrical signal.

It will be appreciated that heart sounds may be considered as oneexample of mechanical indications of the heart beating. Otherillustrative mechanical indications may include, for example,endocardial acceleration or movement of a heart wall detected by anaccelerometer in the LCP, acceleration or movement of a heart walldetected by an accelerometer in the SICD, a pressure, pressure change,or pressure change rate in a chamber of the heart detected by a pressuresensor of the LCP, acoustic signals caused by heart movements detectedby an acoustic sensor (e.g. accelerometer, microphone, etc.), twistingof the heart detected by a gyroscope in the LCP and/or any othersuitable indication of a heart chamber beating.

In some cases, there may be a first heart sound denoted 51 that isproduced by vibrations generated by closure of the mitral and tricuspidvalves during a ventricular contraction, a second heart sound denoted S2that is produced by closure of the aortic and pulmonary valves, a thirdheart sound denoted S3 that is an early diastolic sound caused by therapid entry of blood from the right atrium into the right ventricle andfrom the left atrium into the left ventricle, and a fourth heart sounddenoted S4 that is a late diastolic sound corresponding to lateventricular filling during an active atrial contraction. These aremechanical responses that can be detected using various sensors (e.g.microphone, hydrophone, accelerometer, etc.).

Because the heart sounds are a result of cardiac muscle contracting orrelaxing in response to an electrical signal, it will be appreciatedthat there is a delay between the electrical signal, indicated by theECG 300, and the corresponding mechanical indication, indicated in theexample shown by the heart sounds trace 302. For example, the P-wave ofthe ECG 300 is an electrical signal triggering an atrial contraction.The S4 heart sound is the mechanical signal caused by the atrialcontraction. In some cases, it may be possible to use this relationshipbetween the P-wave and the S4 heart sound. For example, if one of thesesignals can be detected, their expected timing relationship can be usedas a mechanism to search for the other. For example, if the P-wave canbe detected, a window following the P-wave can be defined and searchedin order to help find and/or isolate the corresponding S4 heart sound.In some cases, detection of both signals may be an indication of anincreased confidence level in a detected atrial contraction. In somecases, detection of either signal may be sufficient to identify anatrial contraction. The identification of an atrial contraction may beused to identify an atrial contraction timing fiducial (e.g. a timingmarker of the atrial contraction).

With traditional systems having transvenous leads, the intracardiacelectrodes are placed to detect the atrial depolarization while alsodelivering pacing therapy to one or both ventricles. As a result, thecircuitry of a single device would receive, directly, information forthe P-wave allowing delivery at a timed interval of a pacing pulse toproperly coordinate the ventricular pace with the atrial contraction andimprove pumping efficiency. However, with a system only having an LCPimplanted within a ventricle, it may be difficult to detect therelatively small P-wave from within the ventricle, and as such, it iscontemplated that the LCP may be configured to detect atrial activitywithout relying on the P-wave (e.g. using S4). The detected atrialactivity may be used to identify an atrial timing fiducial that can beused as a basis for timing a pacing pulse in the ventricle (e.g. afteran AV delay).

In some examples, a time window for atrial artifact detection is definedduring which the LCP 100 may specifically look for atrial artifacts(such as, but not limited to, atrial contraction) to determine an atrialtiming fiducial. Such windows may be defined by analysis of the cardiacsignals obtained from a patient using, for example, a detectedventricular event such as the R-wave/QRS complex or the T-wave of aprevious heart beat as the starting point for timing delays 304, 306, asshown in FIG. 4. Timing delays 304, 306 may be dynamic based on theoverall heart beat rate of the patient using data gathered from apatient or using a formula or accepted relationship. Other windows maybe determined based on detected atrial artifacts and/or determinedatrial events, as described in more detail herein.

In some cases, the relationship of certain electrical signals and/ormechanical indications may be used to predict the timing of otherelectrical signals and/or mechanical indications within the sameheartbeat. Alternatively, or in addition, the timing of certainelectrical signals and/or mechanical indications corresponding to aparticular heartbeat may be used to predict the timing of otherelectrical signals and/or mechanical indications within a subsequentheartbeat.

It will be appreciated that as the heart undergoes a cardiac cycle, theblood pressures and blood volumes within the heart vary over time. FIG.5 illustrates how these parameters correlate with the electrical signalsand corresponding mechanical indications. FIG. 5 shows an illustrativeexample of the aortic pressure, left ventricular pressure, left atrialpressure, left ventricular volume, an electrocardiogram (ECG or egram),and heart sounds of the heart over two consecutive heart beats. Acardiac cycle may begin with diastole, and the mitral valve opens. Theventricular pressure falls below the atrial pressure, resulting in theventricle filling with blood. During ventricular filling, the aorticpressure slowly decreases as shown. During systole, the ventriclecontracts. When ventricular pressure exceeds the atrial pressure, themitral valve closes, generating the S1 heart sound. Before the aorticvalve opens, an isovolumetric contraction phase occurs where theventricle pressure rapidly increases but the ventricle volume does notsignificantly change. Once the ventricular pressure equals the aorticpressure, the aortic valve opens and the ejection phase begins whereblood is ejected from the left ventricle into the aorta. The ejectionphase continues until the ventricular pressure falls below the aorticpressure, at which point the aortic valve closes, generating the S2heart sound. At this point, the isovolumetric relaxation phase beginsand ventricular pressure falls rapidly until it is exceeded by theatrial pressure, at which point the mitral valve opens and the cyclerepeats.

Contractions of the atria are initiated near the end of ventriculardiastole. The active atrial contraction pushes or forces additionalvolumes of blood into the ventricles (often referred to as “atrialkick”) in addition to the volumes associated with passive filling. Insome cases, the atrial kick contributes in the range of about 20% of thevolume of blood toward ventricular preload. At normal heart rates, theatrial contractions are considered highly desirable for adequateventricular filling. However, as heart rates increase, atrial fillingbecomes increasingly important for ventricular filling because the timeinterval between contractions for active filling becomes progressivelyshorter. Cardiac pressure curves for the pulmonary artery, the rightatrium, and the right ventricle, and the cardiac volume curve for theright ventricle, may be similar to those illustrated in FIG. 5.Typically, the cardiac pressure in the right ventricle is lower than thecardiac pressure in the left ventricle.

The heart sound signals shown in FIG. 5 can be recorded using acousticsensors, for example a microphone, which may capture the acoustic wavesresulted from such heart sounds. In another example, the heart soundscan be recorded using accelerometers or pressure sensors that capturethe vibrations or pressure waves caused by the heart sounds. The heartsound signals can be recorded within or outside the heart. These arejust examples.

In some cases, sensing atrial events or artifacts indicative of anatrial event may allow a device, such as LCP 100 implanted in theventricle, to detect an atrial contraction, resulting in, for example,an atrial kick. In some cases, signals that provide an indication of anatrial contraction may include one or more of an S3 heart sound signal,an S4 heart sound signal, an A-wave signal (pressure wave) and a P-wavesignal. In some cases, signals that can provide an indication of aventricular contraction may include one or more of an R-wave, aventricle pressure signal, a ventricle change in pressure signal(dP/dt), a ventricle wall acceleration signal, a ventricle twist signal,a blood flow rate signal, and a ventricle volume signal. These are justsome examples.

Some other events or artifacts detected may include, but are not limitedto, S1 heart sounds, S2 heart sounds, ventricular volume, ventricularwall dimension, cardiac tissue and/or blood vibration, atrium toventricle blood movement, ventricular wall and/or atrioventricular (AV)valve position, akinetic pressure, ventricular twist, and any otherevent or artifact suitable for identifying an atrial event, and/orcombinations thereof.

It is contemplated that a number of different sensor modalities may beused to help detect atrial events or artifacts indicative of an atrialevent from the ventricle. FIG. 6 shows a table 320 that includes acolumn for each of various illustrative artifact(s), and a row for eachillustrative sensor modality. An “X” indicates the sensor modalitiesthat may be used to detect the corresponding artifact.

In FIG. 6, it can be seen that voltage may be used to detect P-waves,such as via an electrogram or an electrocardiogram (ECG). It iscontemplated that, in some cases, an LCP implanted in the rightventricle may have a free end (e.g. end that is not affixed to thetissue) pointed towards the tricuspid valve. Due to their anatomicalproximity, the electrodes of the LCP may be used to detect atrialdepolarization (e.g., the p-wave). From the ventricle, the p-wave may berelatively small and difficult to detect. In some cases, the LCP mayidentify a time window around when the p-wave is expected, and the LCPmay increase amplification and/or add special filtering and/or signalaveraging (e.g. see FIG. 11) to help identify the p-wave during thewindow. Alternatively, or in addition, the p-wave may be detected alongwith one or more other artifacts to help confirm an atrial contractionand to develop an atrial timing fiducial therefrom.

As shown in FIG. 6, pressure may be used to identify a number ofdifferent atrial artifacts. For example DC and/or near DC type pressuremeasurements (e.g. 0-10 Hz range) may be used to identify passivefilling of the ventricle (e.g., akinetic pressure). Low frequency (e.g.1-5 Hz range) AC type pressure measurements may be used to detect theA-wave (atrial pressure wave in the ventricle), while higher frequency(e.g. 15-30 Hz range) AC type pressure measurements may be used todetect heart sounds. These are just examples. In some cases, pressuremay be used to identify the transition between passive and activefilling modes. This transition may be used as an indicator of atrialcontraction. Other suitable methods for measuring or detecting pressurein one or more heart chambers may also be used, as desired. Someillustrative but non-limiting pressure sensors and configurations forsensing pressure using an LCP are described in commonly assigned PatentApplication No. 62/413,766 entitled “IMPLANTABLE MEDICAL DEVICE WITHPRESSURE SENSOR and filed on Oct. 27, 2016, and Patent Application No.62/547,458, entitled IMPLANTABLE MEDICAL DEVICE WITH PRESSURE SENSOR andfiled on Aug. 18, 2017, which are hereby incorporated by reference.

As shown in FIG. 6, impedance measurements may be used to determineventricular volume changes which may then be used to infer a pressurewave (e.g. A-wave) due to an atrial contraction. In one example, as thevolume of blood in the ventricle changes, the impedance between theelectrodes of the LCP changes. It is contemplated that the rate ofchange in the volume (e.g., an increase in the rate of blood enteringthe ventricle and hence a faster change in volume of the ventricle) maybe used to identify the start of active filling and thus an atrialcontraction. Some illustrative uses of impedance measurements in theheart are described in commonly assigned patent application Ser. No.15/630,677 entitled LEADLESS CARDIAC PACEMAKER FOR GENERATING CARDIACPRESSURE-VOLUME LOOP and filed on Jun. 22, 2017, which is herebyincorporated by reference.

As blood enters the ventricle as a result of an atrial contraction, theventricle may stretch. The stretching of the ventricle may be measurewith a strain sensor. A strain sensor may require two or more points offixation. Acceleration may be used to measure contractility of the heartH, as well as sounds. In some cases, cardiac output can be determinedwhen acceleration measurements are combined with ventricle pressure,cardiac volume and/or other sensed parameters.

It should be understood that the table 320 shown in FIG. 6 is notintended to include every possible artifact or sensor modality fordetecting each artifact. Those of skill in the art will recognize thatother artifacts, sensor modalities and/or combinations thereof may beused to identify an atrial event from the ventricle. In one additionalexample, a respiratory phase sensor may be used with other atrialartifacts described herein or by itself to help identify an atrialartifact.

The atrial event and/or artifacts indicative of an atrial event mayoccur during either or both passive ventricular filling or activeventricular filling. FIG. 7 illustrates a table 330 of the cardiacphases, and the artifact(s) that may occur during that phases of thecardiac cycle, where an “X” is used to denote that the correspondingartifact occurs during the identified cardiac phase. Due to anelectromechanical delay, the initial portion of the P-wave may fall intothe passive filling phase while the later portion may fall into theactive filling phase, and that is why an “X” is in both rows of thetable 330. Although not required, it is contemplated that force per unitarea type measurements may be provided as a DC voltage or current and/ora low frequency pressure signal linearly proportional to pressure. Soundtype pressure measurements (e.g., infrasonic and sonic) may be providedas an AC pressure.

In some instances, ultrasound may use a combined ultrasound source andsensor, although this is not required. The source and sensor may beseparately provided, as desired. It is contemplated that ultrasoundimaging may be used in a device implanted in the ventricle to see theatrial wall (e.g., through the tricuspid valve), tricuspid closing,and/or a flow increase due to an atrial contraction to help identify anA-wave. In some cases, ultrasound sensor may detect an atrial arrhythmia(e.g. atrial flutter or atrial fibrillation). During normal sinus rhythm(NSR) atrial blood flow into the ventricle is comprised of twosequential components, an E (early) wave followed by an A (atrial) wave.During atrial arrhythmias the E wave is largely unchanged from that inNSR, however the A wave is either missing (atrial fibrillation) orsmaller and much faster (atrial flutter). During a detected atrialarrhythmia an LCP with an ultrasound sensor may modify its behavior(e.g. revert from VVD mode to VVI mode).

It should be noted that while the heart sounds are indicated as capableof being identified with an accelerometer, the accelerometer actuallymeasures or detects mechanical vibration associated with the heart soundand not the pressure of the sound waves. In some cases, the measuredartifact may not occur distinctly within one cardiac phase or another.For example, ventricular twist may be used to identify the end of activeventricular filling (e.g., ejection). Further, the S1 heart sound mayoccur at the end of active ventricular filling, while the S2 heart soundmay occur shortly before the beginning of passive ventricular filling.These are just some examples.

In some cases, the LCP 100 may be configured to determine an atrialcontraction timing fiducial based at least in part upon a sensedindication of an atrial contraction in a first heartbeat and/or a sensedindication of a ventricular contraction in the first heartbeat and/orone or more immediately preceding heartbeat(s). In some cases, theprocessing module 110 of the LCP 100 may be configured to generate anddeliver a ventricle pacing pulse using the determined atrial contractiontiming fiducial (e.g. after an A-V delay).

As described above, atrial events or artifacts indicative of an atrialevent may be used by an LCP in the ventricle (e.g. right ventricle) totime a pacing pulse for the ventricle in support of treating bradycardiaevents. In some cases, the timing of the ventricle pacing pulse may beadjusted to improve the amount of blood entering the right ventriclethrough active filling. In some instances, this may include adjusting anAV delay relative to an atrial fiducial (e.g., atrial kick). In somecases, a measured pressure change (or other atrial fiducial) over timemay be used to support management of a CRT cardiac therapy (if placed inthe left ventricle), patient health status monitoring and/or any othersuitable goal. It is contemplated that detecting events in one of orboth of the ventricle and atrium using a single LCP implanted in theventricle may replicate a dual chamber system with only a single device.That is, a single device positioned in the ventricle may listening toboth the ventricle and the atrium and pacing accordingly (e.g., a VDDdevice).

FIG. 8 is a side view of an illustrative implantable leadless cardiacpacemaker (LCP) 400 which may be positioned within the ventricle andconfigured to listen to both the ventricle and the atrium. The LCP 400may be similar in form and function to the LCP 100 described above. TheLCP 400 may include any of the sensing, electrical, control, and/orpacing modules and/or structural features described herein. The LCP 400may include a shell or housing 402 having a proximal end 404 and adistal end 406. The illustrative LCP 400 includes a first electrode 410secured relative to the housing 402 and positioned adjacent to thedistal end 406 of the housing 402 and a second electrode 412 securedrelative to the housing 402 and positioned adjacent to the proximal end404 of the housing 402. In some cases, the housing 402 may include aconductive material and may be insulated along a portion of its length.A section along the proximal end 404 may be free of insulation so as todefine the second electrode 412. The electrodes 410, 412 may be sensingand/or pacing electrodes to provide electro-therapy and/or sensingcapabilities. The first electrode 410 may be capable of being positionedagainst or otherwise in contact with the cardiac tissue of the heart,while the second electrode 412 may be spaced away from the firstelectrode 410. The first and/or second electrodes 410, 412 may beexposed to the environment outside the housing 402 (e.g., to bloodand/or tissue).

It is contemplated that the housing 402 may take a variety of differentshapes. For example, in some cases, the housing 402 may have a generallycylindrical shape. In other cases, the housing 402 may have a half-domeshape. In yet other embodiments, the housing 402 may be a rectangularprism. It is contemplated that the housing may take any cross sectionalshape desired, including but not limited to annular, polygonal, oblong,square, etc.

In some cases, the LCP 400 may include a pulse generator (e.g.,electrical circuitry) and a power source (e.g., a battery) within thehousing 402 to provide electrical signals to the electrodes 410, 412 tocontrol the pacing/sensing electrodes 410, 412. While not explicitlyshown, the LCP 400 may also include a communications module, anelectrical sensing module, a mechanical sensing module, and/or aprocessing module, and the associated circuitry, similar in form andfunction to the modules 102, 106, 108, 110 described above. The variousmodules and electrical circuitry may be disposed within the housing 402.Electrical communication between the pulse generator and the electrodes410, 412 may provide electrical stimulation to heart tissue and/or sensea physiological condition.

In the example shown, the LCP 400 includes a fixation mechanism 414proximate the distal end 406 of the housing 402. The fixation mechanism414 is configured to attach the LCP 400 to a wall of the heart H, orotherwise anchor the LCP 400 to the anatomy of the patient. As shown inFIG. 8, in some instances, the fixation mechanism 414 may include one ormore, or a plurality of hooks or tines 416 anchored into the cardiactissue of the heart H to attach the LCP 400 to a tissue wall. In otherinstances, the fixation mechanism 414 may include one or more, or aplurality of passive tines, configured to entangle with trabeculaewithin the chamber of the heart H and/or a helical fixation anchorconfigured to be screwed into a tissue wall to anchor the LCP 400 to theheart H. These are just examples.

The LCP 400 may further include a docking member 420 proximate theproximal end 404 of the housing 402. The docking member 420 may beconfigured to facilitate delivery and/or retrieval of the LCP 400. Forexample, the docking member 420 may extend from the proximal end 404 ofthe housing 402 along a longitudinal axis of the housing 402. Thedocking member 420 may include a head portion 422 and a neck portion 424extending between the housing 402 and the head portion 422. The headportion 422 may be an enlarged portion relative to the neck portion 424.For example, the head portion 422 may have a radial dimension from thelongitudinal axis of the LCP 400 that is greater than a radial dimensionof the neck portion 424 from the longitudinal axis of the LCP 400. Insome cases, the docking member 420 may further include a tetherretention structure 426 extending from or recessed within the headportion 422. The tether retention structure may define an opening 428configured to receive a tether or other anchoring mechanismtherethrough. The retention structure may take any shape that providesan enclosed perimeter surrounding the opening such that a tether may besecurably and releasably passed (e.g., looped) through the opening 428.In some cases, the retention structure may extend though the headportion 422, along the neck portion 424, and to or into the proximal end404 of the housing 402. The docking member 420 may be configured tofacilitate delivery of the LCP 400 to the intracardiac site and/orretrieval of the LCP 400 from the intracardiac site. While thisdescribes one example docking member 420, it is contemplated that thedocking member 420, when provided, can have any suitable configuration.

It is contemplated that the LCP 400 may include one or more sensors 430coupled to or formed within the housing 402 such that the sensor(s) isexposed to and/or otherwise operationally coupled with (e.g., responsiveto) the environment outside the housing 402 to measure or detect variousartifacts within the heart. The one or more sensors 430 may be of a samemodality or a combination of two or more different sensing modalities,as desired. For example, the one or more sensors 430 may be use voltage,pressure, sound, ultrasound, impedance, strain, acceleration, flow,and/or rotation to detect P-waves, A-waves, S1-S4 heart sounds,ventricular volume, ventricular wall dimensions, cardiac tissue and/orblood vibration, atrium to ventricle blood movement, ventricular walland/or atrioventricular valve position, akinetic pressure, and/orventricular twist, such as described with respect to FIGS. 6 and 7. Thesensors may be a part of, coupled to, and/or in electrical communicationwith a sensing module disposed within the housing 402. In addition tosensing artifacts within the heart, the sensing module may be furtherconfigured to detect physiological conditions that may impact the LCP'sability to detect artifacts including, but not limited to posture,activity and/or respiration. The use of two or more sensors incombination may allow for the removal of some common mode noise (e.g.,may eliminate gross body motion).

In some cases, the one or more sensors 430 may be coupled to an exteriorsurface of the housing 402. In other cases, the one or more sensors 430may be positioned within the housing 402 with an artifact acting on thehousing and/or a port on the housing 402 to affect the sensor 430. Inone illustrative example, if the LCP 400 is placed in the rightventricle, the sensor(s) 430 may be a pressure sensor configured tomeasure a pressure within the right ventricle. If the LCP 400 is placedin another portion of the heart (such as one of the atriums or the leftventricle), the pressures sensor(s) may measure the pressure within thatportion of the heart. In some cases, the sensor(s) 430 may be sensitiveenough to detect an artifact in a heart chamber different from thechamber in which the LCP 400 is positioned. For example, in someinstances a sensor 430 may detect a pressure change caused by an atrialcontraction (e.g., atrial kick) when the LCP 400 is placed in the rightventricle. Some illustrative sensor configurations will be described inmore detail herein.

FIG. 9A is a plan view of the example leadless cardiac pacing device 400implanted within a right ventricle RV of the heart H during ventricularfilling. The right atrium RA, left ventricle LV, left atrium LA, andaorta A are also illustrated. FIG. 9B is a plan view of the leadlesscardiac pacing device 610 implanted within a right ventricle of theheart H during ventricular contraction. These figures illustrate how thevolume of the right ventricle may change over a cardiac cycle. As can beseen in FIGS. 9A and 9B, the volume of the right ventricle duringventricular filling is larger than the volume of the right ventricle ofthe heart after ventricular contraction.

While it is desirable to identify an atrial contraction often associatedwith the A-wave, the A-wave can be difficult to detect as it may be verysmall in magnitude and detection of it may come and go. It iscontemplated that a combination of sensor modalities and/or measuredatrial artifacts may be used to identify an atrial timing fiducial. Forexample, it is contemplated that any of the sensor modalities identifiedwith respect to FIGS. 6 and 7 may be combined with any other sensormodality to identify an atrial timing fiducial. In some cases, apressure signal may be used to determine a number of parameters. Forexample, a pressure signal may be used to determine or detect an A-wave(atrial kick). In another example, the pressure signal may be used todetermine or detect a pressure pulse or pressure vibrations associatedwith S4, which may, for example, be in the 15-30 Hz range. In somecases, the S4 heart sound may be easier to detect using a pressuresignal from a pressure sensor than from an accelerometer signal fromaccelerometer or using an acoustic signal from an acoustic sensor,particularly since the ventricular pressure is not changingsubstantially during this time period (ventricle is filling) and sincethere may be a great deal of unwanted signal (i.e. noise) in theaccelerometer signal due to patient activity. In another example, apressure signal may be used to determine a change in ventricle pressurerelative to time (dP/dt).

In some cases, the circuitry and/or processing module of the LCP 400 mayalso be configured determine an atrial contraction timing fiducial basedat least in part upon two or more of a signal received from theelectrical sensing module, mechanical sensing module, and/orcommunication module. In some cases, the electrical cardiac signalreceived via the electrode arrangement 410, 412 may include at least aportion of an electrocardiogram (ECG). In some cases, the electricalcardiac signal received via electrode arrangement 410, 412 may include aP-wave. In some instances, the electrical cardiac signal received viathe electrode arrangement 410, 412 may include a QRS complex, from whicha QRS width can be determined. In some cases, the electrical cardiacsignal received via electrode arrangement 410, 412 may include twoconsecutive R waves, from which an R-wave to R-wave interval can bedetermined. In some cases, the electrical cardiac signal may include aconducted or other communicated electrical signal from another device(e.g. SICD device) that includes an indication of an atrial or othercontraction of the heart H. In some cases, the processing module and/orcircuity may be configured to generate and deliver a ventricle pacingpulse using the atrial contraction timing fiducial.

It is contemplated that the use of sensors to determine an atrialcontraction timing fiducial without having to detect the A-wave mayallow the LCP 100, 400 to predict or recognize when an A-wave likelyoccurred, even when the A-wave itself was not detected. The predictedtime of the A-wave may then be used as an atrial contraction timingfiducial for pacing the ventricle. The A-wave may be particularlydifficult to detect when, for example, the heart is experiencing atrialfibrillation, a patient is in certain postures, the respiration rate ishigh, the patient activity is high, the heart rate is high, the atriaare hypocontractile or akinetic, and/or during periods of high heartrate variability (HRV).

In the cardiac cycle, the ventricles receive blood from the atria firstthrough passive filling and then through active filling. Discussion ofpassive and active filling will be described with reference to a rightside of the heart, however, it should be understood that a similarprocess is occurring in the left side of the heart. Passive filling ofthe right ventricle begins when the there is a pressure gradient betweenthe chambers causing the tricuspid valve to open and blood accumulatedin the right atrium to flow into the right ventricle. Both the rightatrium and the right ventricle continue to fill as blood returns to theheart. The right atrium contracts near the end of ventricular diastole.Atrial depolarization begins at the P-wave of the electrocardiogram. Asa result of the P-wave, atrial cells develop tension and shorteningcausing the atrial pressure to increase (e.g. A-wave). These activecontraction forces force additional volumes of blood into the ventricle(often referred to as the “atrial kick”). The active contraction forcesbegin the active filling phase. At normal heart rates, the atrialcontractions are considered desirable for adequate ventricular filling.As heart rate increases, atrial filling becomes increasingly importantfor ventricular filling because the time interval between contractionsfor filling becomes progressively shorter. Atrial fibrillation and/orasynchronized atrial-ventricular contractions can result in a minimalcontribution to preload via atrial contraction.

As described above, the fourth heart sound (e.g., S4) is typically agallop sound that results from a forceful atrial contraction duringpresystole that ejected blood into a ventricle which cannot expandfurther. The fourth heart sound occurs during the last one-third ofdiastole about 90 milliseconds before S1. The frequency of S4 may be inthe range of about 15 Hertz (Hz) to about 30 Hz, although the frequencymay sometimes be outside this range. Due to the low pitch, S4 (andsometimes S3) are usually not audible with a typical stethoscope. It iscontemplated that the S4 heart sound may be used to identify the startof active filling of the ventricle. In some cases, the processing module110 and/or circuitry may be programmed to begin looking for the S4 heartsound just before the S1 heart sound is expected (projected from one ormore previous heart beats).

The heart sounds may be time dependent on the heart rate in manner thatchanges linearly with the heart rate. For example, as the heart rateincreases, the time between the heart sounds (e.g. S1 to S1; S4 to S1,etc.) may decrease in a linear and predicable manner. This may allow theS4 heart sound to be used to identify a reliable atrial event and/or asan atrial timing fiducial over a range of heart rates.

As noted above, the S4 heart sound may be identified and/or detectedusing a variety of different sensors, including but not limited to ahigher frequency pressure sensor (e.g. 15 to 30 Hz), a hydrophone, amicrophone, and/or an accelerometer. These are just some examples of howthe LCP 400 can detect an artifact during active ventricular filling andidentify an atrial timing fiducial based on the detected artifact.

While the above example was described with respect to active filling, itis contemplated that an artifact identified during passive filling mayalso be used to identify an atrial event which may then be used toidentify an atrial timing fiducial. For example, the third heart sound(e.g., S3) occurs near the middle of passive filling. Passive fillingmay generate a very low frequency sound (in the range of 0 to 10 Hz)which may be detected by a DC capable pressure sensor. This may allowthe S3 heart sound to be used to identify an atrial event and/or as anatrial timing fiducial over a range of heart rates.

FIG. 10 is a flow diagram showing an illustrative method 500 ofgenerating a ventricular pulse using an LCP that is disposed with theright ventricle. In some cases, as indicated at block 502, a firstsignal (e.g., an atrial artifact) indicating an atrial event of apatient's heart may be sensed with a sensing module of the LCP. A seconddifferent signal related to the atrial event of the patient's heart mayalso be detected, as indicated at block 504. The second different signalmay be sensed by the LCP, or may be received from another device (e.g.an SICD or another LCP) via a communication module of the LCP.

In some instances, the first signal and/or the second signal may begenerated via one or more sensors within or on the housing the LCP. Asdescribed above, the sensing module of the LCP 400 may sense differentevents depending on whether attempting to identify active filling orpassive filling (see FIG. 7). Some illustrative sensing modalities forsensing active filling may include, but are not limited to impedance,strain, sound, rotation, or flow, any or all of which may be used todetect at least one of a P-wave, S2 heart sound, S3 heart sound,ventricular volume, ventricular wall dimension, ventricular bloodmovement, ventricular wall movement, tricuspid valve position, mitralvalve position, and/or akinetic ventricular pressure. Some illustrativesensing modalities for sensing passive filling may include, but are notlimited to pressure, impedance, strain, sound, rotation, acceleration,voltage, and flow, which may be used to detect at least one of a P-wave,A-wave, S1 heart sound, S4 heart sound, ventricular volume, ventricularwall dimension, cardiac tissue vibration, ventricular blood movement,ventricular wall movement, tricuspid valve position, and mitral valveposition.

The circuitry within the LCP 400 may be configured to determine anatrial timing fiducial based at least in part on the first and/or secondsensed signals, as indicated at block 506. For example, the circuitrymay be configured to determine when the A-wave occurs based on a sensedS4 heart sound and/or other atrial artifact. This is just one example.Those skilled in the art will recognize that any number of artifacts (orcombinations thereof) can be used to determine an atrial timingfiducial. The circuitry may be configured to then generate and deliver aventricular pacing pulse using the determined atrial timing fiducial, asindicated at block 508. The control circuitry may delay delivering apacing pulse to the ventricle until an appropriate AV delay expiresafter the determined atrial timing fiducial. Notably, a different AVdelay may be used for different atrial timing fiducials (see FIG. 14).

While the control timing of the pacing pulse may be triggered by anatrial timing fiducial that is based on arterial artifacts detectedduring a single heart beat, it is contemplated that the pacing pulse maybe triggered by an atrial timing fiducial that is based on arterialand/or other artifacts detected during two or more previous heart beats.

In some cases, the LCP may determine an average timing for a particularatrial artifact and/or atrial timing fiducial over multiple heart beats.

The circuitry of the LCP may further be configured to determineintrinsic intervals within the cardiac cycle. This capability may beprovided within the control circuitry or provided as a separate intervaldetermination module in the LCP. In some cases, the circuitry may beconfigured to identify intrinsic intervals including atrial to atrialevent or artifact intervals, atrial to ventricle event or artifactintervals, ventricle to atrial event or artifact intervals, and/orventricle to ventricle event or artifact intervals. This information maybe useful in predicting when, for example, an atrial event (e.g. A-wave)is expected to occur. This may be useful in, for example, confirming anatrial event that is sensed by the LCP. This may also be useful inidentifying a window of time around which an atrial event is expected,such that the LCP may increase amplification and/or add specialfiltering and/or signal averaging (e.g. see FIG. 11) to help identifythe atrial event during the window.

In some cases, the sensing module of the LCP may be configured tomanipulate the signal prior to identifying an atrial event. For example,the sensing module may include one or more filters for filtering asignal. In some cases, the filter may include a first filter for passinga first frequency band, a second filter for passing a second frequencyband, and a third filter for passing a third frequency band. The filtermay include more than three frequency bands or fewer than threefrequency bands, as desired. In some cases, the filter may be band-passfilter, a low pass filter, a high pass filter, and/or any other suitablefilter. In some cases, a band-pass filter may be in the range of 1 to 5Hz. In other cases, a bandpass filter may be in the range of 15 to 30Hz. In yet another example, the filter may be a low-pass filter in therange of 0 to 10 Hz. These are just examples; other frequency ranges canbe used, as desired. Also, filters may be employed that are not based onfrequency, but rather some other signal feature such as amplitude,phase, etc.

In some cases it may be desirable to limit the time frame in which theLCP 400 is looking for an atrial artifact. For example, battery life maybe increased when the circuitry is searching for an artifact only duringa limited window or period of time that is less than an entire cardiaccycle. The method for determining a time window for searching for anatrial artifact may include first identifying an expected time frame forthe atrial event (e.g., atrial contraction) and then defining a searchwindow accordingly. Referring to FIG. 11, to begin, the control modulemay select one or more signals with a desirable characteristic for afirst timing fiducial signal to use as a time reference. The signal maybe one or more of a pressure signal, an acoustic single, an accelerationsignal, an electrical signal, etc. It is contemplated that the fiducialsignal may be a different signal from the signal used to identify anatrial artifact and hence an atrial event or atrial timing fiducial. Inthe example shown in FIG. 12, the selected signal may be an ECG 554generated from electrical signals in the right ventricle. Within the ECG554, a specific feature, such as, but not limited to the R-wave may beselected as the fiducial reference feature 556. The ECGs 554 signals fora plurality of cardiac cycles (e.g., at least two or more) may beaveraged, with the fiducial reference features 556 in each ECG 554aligned. This signal averaging technique may help reveal small signalsby canceling out random noise. The signal averaging technique may alsobe used to identify various cardiac events, atrial event templates,appropriate A-V delays for a variety of different atrial timingfiducials (e.g. A-wave, P-wave, R-wave, and/or other atrial timingfiducial).

A window 560 where an atrial event is expected to occur can then beisolated. For example, an atrial event (e.g., atrial contraction) may beexpected to occur within a time window 560 before the next R-wave. Usingthis time window 560, the LCP may search for the atrial event. In somecases, the LCP may increase amplification and/or add special filteringand/or signal averaging to help identify the atrial event during thetime window 560. In some case, the window 560 can be used as a referencepoint for determining another window in which another signal should berecorded and searched to identify an atrial artifact from which anatrial event can be deduced.

In some cases, the timing window for identifying an atrial contractionmay be based on artifacts occurring during passive filling of theventricle. In some cases the down-stroke of the ventricular pressure(e.g., when the A-V valve opens) may be used to open a timing window fordetecting an atrial artifact and/or atrial contraction. An upslope inventricular pressure may trigger an open sensing window to detect theatrial kick. FIG. 12 illustrates a portion of the pressure profile 600of the right ventricle relative to the S3 and S4 heart sounds. The rightventricle may have an increase in pressure at the start of systole. Thepressure may decrease as blood exits the ventricle. This sharp decreasein pressure may signal the control module to open a search window. Forexample, the search window may be opened in the general time frameindicated by box 602. This may command the control module to beginsearching for an atrial artifact that may be used to start a timingwindow. The timing window 604 may open at the S3 heart sound 606 andclose at the R-wave 608. The S4 heart sound and the atrial kick mayoccur within this timing window, as shown. It is contemplated that thecontrol module may utilize automatic gain control to increase thesensitivity (e.g., reduce the threshold and/or increase the gain) overthe period of the timing window to help increase the sensitivity whenthe expected event (e.g., atrial kick) is expected to occur.

In another example, the S2 heart sound may be used to identify the startof passive filling. It is contemplated that a pressure sensor in the LCPmay be used to detect the pressure change associated with the atrialkick, or any of the atrial artifacts identified herein can be usedeither alone or in combination with the atrial kick as the atrial timingfiducial. The LCP 100, 400 can then pace the ventricle based off of theartifact, the atrial kick or a combination thereof. In another example,ventricular impedance may be used to identify volume changes in theventricle, which may then be used to infer a pressure wave due to theatrial contraction. In another example, one or more atrial artifacts maybe used to identify the end of passive filling for hemodynamicoptimization. For example, passive filling may by typically completedapproximately 500 milliseconds after the S2 heart sound. In yet anotherexample, the timing window may be open between the S3 and S4 heartsounds. In some cases, the control module of the LCP may set adecreasing signal threshold to allow smaller signals to reach the inputamplifier after the S3 heart sound in order to increase the signal. Insome cases, the control module may be configured to run a continuousintegration of the pressure signal as a surrogate for pressure, whichmay then be used to create a timing window. It is contemplated thatchanges in ventricular filling and/or pressures over time may be used topick up respiration signals that may be used to support other featuresof the LCP 100, 400. These are just some examples of how atrialartifacts can be detected by an LCP within the ventricle, which can thenbe used to identify an atrial timing fiducial for use in timing deliveryof a pacing pulse to the ventricle.

It is contemplated that the control module of the LCP 100, 400 may beconfigured to search for an atrial artifact and identify a search windowin more than one manner. In some cases, pacing can cover up, hide, orotherwise distort atrial artifacts and may make then difficult toidentify. It may be desirable to allow the LCP to enter a listening modein which the control circuitry does not issue pacing commands. Thelistening mode may be for a predefined window of time during a cardiaccycle that is less than the entire cardiac cycle. This may allow the LCP400 to identify an atrial event without hiding or covering up the atrialartifact of interest (e.g. the A-wave). In some cases, such as when thepatient is not pacing dependent, pacing can be paused for a cardiaccycle or two when no atrial activity is detected in order to determineif pacing is covering the atrial artifact(s) if interest. If the patientis pacing dependent, the pacing rate may be slowed (period extended) toallow for a larger period of time to search for the atrial artifactswithout a pacing pulse present. Once the atrial artifact has beenidentified, the LCP may use the artifact to control the timing of thepacing pulse for a one or more cardiac cycles. In the event that anatrial artifact is not found, the LCP may return to its original pacingrate. In some cases, the LCP may be configured to pause or delay pacingand look for an atrial artifact and/or event on a predetermined timeschedule.

It is further contemplated that in the event that the atrial artifact isnot found the control module may be configured to deliver a pacingtherapy at an altered pacing rate. In an example, the altered pacingrate may be less than the pacing rate delivered while atrial events aredetected. In another example, the altered pacing rate may be greaterthan the pacing rate delivered while atrial events are detected. In afurther example, the altered pacing rate may be static (e.g., remainconstant) during the time there is a failure to detect atrial events. Inyet another example, the altered pacing rate may be dynamic (e.g.,change) during the time there is a failure to detect atrial events.

In another example, the control module may be configured to switch to apacing only mode (in some cases a VOO mode). In this example, thecontrol module may be configured to analyze the inputs received from thevarious sensor modules to determine if some sensors are providing aclearer signal than others. The control module may be configured toprioritize which sensor module is used to search for an atrial artifactand/or event before re-entering VDD mode. When in VOO mode, it may bedesirable to pace off of the P-wave. However if this is not possible, itmay be desirable to open a timing window based on other sensorsincluding, but not limited to, pressure sensors and/or accelerometers toidentify an atrial contraction. It is contemplated that the controlmodule may be configured to switch between sensing modes and pacingmodes as needed.

The control module may be configured to determine a quality thresholdfor a timing window, which may reflect the quality of the atrialartifact signal identified during the timing window. For example, thecontrol module may be configured to analyze or grade a current A-wavetiming window. If the current A-wave timing window does not meet certainquality metrics (e.g. percent of cardiac cycles in which an A-Wave isdetected, the signal-to-noise ration of the detected A-wave signal,etc.), the control module may discard the window and use a previouswindow or calculate a new timing window. In some cases, the controlmodule may prioritize one type of atrial artifact over another based onthe quality of the detected signal.

As described above, the LCP 100, 400 may use different atrial and/orventricle artifacts to determine when to search for an artifact and whento open the timing window. The LCP 100, 400 may include a sensing modulethat includes at least two of a pressure measurement module, an acousticmeasurement module, an acceleration measurement module, and anelectrogram measurement module. In some cases, the sensing module mayinclude at least a pressure measurement module and at least one of anacoustic measurement module, an acceleration measurement module, and anelectrogram measurement module. In some cases, the control module mayuse a ventricular event such as the R-wave to identify when to start asearch window. In some cases, the control module may use differentsearch windows to identify atrial artifacts from different measurementmodules. The control module may identify a window of time during each ofone or more cardiac cycles in which an atrial artifact and/or atrialevent is expected to occur. The window of time may be less than anentire cardiac cycle. The control module may analyze informationgathered by the sensing module (e.g., using at least one of the pressuremeasurement module, an acoustic measurement module, an accelerationmeasurement module, and an electrogram measurement module) to identifyan atrial event (e.g., atrial kick). The control module may then deliveror command a pacing module to deliver a ventricular pacing pulse via thepacing electrodes of the LCP. The ventricular pacing pulse is deliveredat a time that is based at least in part on the timing of the identifiedatrial event. For example, the pacing pulse may be delivered apredetermined length of time (e.g. A-V delay) after the identifiedatrial event. It is contemplated that the A-V delay that is used maydepend on the particular atrial event that was identified. That is,different atrial events may cause different A-V delays to be applied.

The control module may be further configured to average the signalsgathered from the sensing module in a similar manner to that describedwith respect to FIG. 11. For example, the control module may beconfigured to use signal averaging of the signals gather at the sensingmodule during each of a plurality of cardiac cycles to determine asignal average. The signal average may then be used to identify a windowof time within a cardiac cycle. The identified window of time may thenbe used in subsequent cardiac cycles to search for and identify anatrial artifact and/or atrial event.

In some cases, the control module may be configured to move the windowof time to search for an atrial artifact and/or atrial event. Forexample, if one of measurement modules of the sensor module is providinga better signal (e.g. better SNR), the control module may base thewindow around the detected artifact with a clearer signal. As theartifacts can occur at varying time points within the cardiac cycle, thewindow may be moved accordingly, sometimes cycle-to-cycle. The controlmodule may be configured to select which measurement module to usedynamically or on a case to case basis.

In some cases, the control module may use different quality measurementsto determine which measurement module to use. For example, the controlmodule may select the measurement module with a better Signal-to-NoiseRatio (SNR). In another example, a p-wave detecting atrial activation byan electrogram measurement module may have a higher priority than apressure signal detecting an atrial kick by a pressure measurementmodule. However, due to the inability of a ventricle only configurationto reliably sense a P-wave, the LCP may not rely solely on the P-wave toidentify an atrial artifact and/or event. It may in fact, it may switchto detecting the A-wave when the P-wave is not available, and/or may usethe A-wave to confirm the detection of a noisy P-wave. These are justexamples.

In some cases, the control module may combine information gathered frommore than one measurement module in identifying an atrial artifactand/or atrial event. For example, the control module may use bothpressure data and electrocardiogram data in identifying an atrialartifact and/or event. In some cases, when data is used from two or moremeasurement modules, the data from each measurement module may beweighted differently (e.g., one may count more or be weighted moreheavily than another). It is further contemplated that the controlmodule may be configured to lengthen the window (e.g., make it longer)under certain conditions. For example, the window may not be long enoughto identify an atrial artifact and/or event or a pacing pulse may becovering up the atrial event. In other cases, the window may beshortened (e.g., when noise is present, the noise may be reduced with ashortened window).

As described herein, the timing intervals for both searching and pacingmay be based on pressure and/or heart sound fiducials (as well as otheratrial artifacts described herein) as opposed to basing the intervalsoff solely of an electrocardiogram. FIG. 13 is a graph 650 ofillustrative cardiac signals including heart sounds, right ventricularpressure, and an electrocardiogram. FIG. 13 also shows various intervalsbetween various artifacts of these signals. It is contemplated that anumber of different artifacts or characteristics during a cardiac cyclecan be used to form a number of different timing intervals. For example,there can be intervals that extend between two electrocardiogram signals(E-E) such as between an R-wave amplitude 670 of a first cardiac cycleand an R-wave amplitude 672 of the next cardiac cycle, as indicated atarrow 652. Another interval may be defined between two pressure signals(P-P), such as between an A-wave pressure 674 and a maximum systolicpressure 676 of the same cardiac cycle, as shown at arrow 654, orbetween a maximum systolic pressure 676 of a first cardiac cycle and anA-wave pressure 678 in the subsequent cardiac cycle, as shown at arrow656. Another illustrative interval may be defined between two acousticsignals (A-A), such as between an S1 heart sound 680 and an S2 heartsound 682, as shown at arrow 658, between an S2 heart sound 682 and anS3 heart sound 684 as shown at arrow 660, and/or between an S3 heartsound 684 and an S1 heart sound 686 of a subsequent cardiac cycle, asshown at arrow 662.

As illustrated in FIG. 13, there can also be intervals defined betweenan electrocardiogram signal and a pressure signal (E-P), between apressure signal and an electrocardiogram signal (P-E), between anelectrocardiogram signal and an acoustic signal (E-A), between anacoustic signal and an electrocardiogram signal (A-E), between apressure signal and an acoustic signal (P-A), and/or between an acousticsignal and a pressure signal (A-P). It is contemplated that anymeasurable parameter may serve as the beginning and/or end of aninterval as desired, and the intervals are not limited to thoseexplicitly described or shown in FIG. 13.

FIG. 14 is a graph 700 of illustrative cardiac signals including heartsounds, right ventricular pressure, and an electrocardiogram. FIG. 14also shows various intervals between various artifacts of these signals.As described herein, there can be a number of different intervals usingvarious sensed parameters. Not only can there be various intervals froma sensed artifact to another sensed artifact, but also various intervalsfrom a sensed artifact to a pacing pulse.

The E-E (R-wave to subsequent R-wave), A-A (S1 to subsequent S1), andP-P (max pressure to subsequent max pressure) intervals shown at 702 arethree ventricular intervals. The E-E (P-wave to subsequent P-wave), A-A(S4 to subsequent S4), and P-P (atrial kick to subsequent atrial kick)intervals shown at 704 are three atrial intervals. These intrinsic samechamber intervals 706 have the same or roughly the same time intervalbetween same sensed artifacts or events regardless of which parameter isused (e.g., R-wave to R-wave, S1 to S1, max pressure to max pressure).In contrast, intervals between chambers 708 vary substantially. As canbe seen at 708 in FIG. 14, atrioventricular (A-V) intervals varysignificantly depending on which atrial event is selected for the atrialtiming fiducial. An E-E (P-wave to R-wave) interval, A-E (S4 to R-wave),and P-E (atrial kick to R-wave) intervals shown at 708 are threeillustrative atrioventricular (AV) intervals each having a differentduration. The duration of each of these AV intervals can be sensedduring one or more intrinsic heart beats (e.g. no pacing). In somecases, the duration of each of these intervals can be sensed during aplurality of intrinsic heart beats (no pacing) and then averaged,resulting in an average AV interval for each of the different atrialtiming fiducials as shown at 710.

As described above, a P-wave may not be consistently detected in adevice implanted in the ventricle. As such, it may be desirable to timethe ventricular pacing pulse (Vp) using a pressure artifact (e.g.,a-wave or atrial kick) as the atrial timing fiducial along with acorresponding AV interval. In another example, it may be desirable totime the ventricular pacing pulse (Vp) using an acoustic artifact (e.g.,S4) as the atrial timing fiducial along with a corresponding AVinterval. The corresponding AV interval used with an acoustic artifact(e.g., S4) may be different than the AV interval used with a pressureartifact, as seen at 710 in FIG. 14. In yet another example, it may bedesirable to time the ventricular pacing pulse (Vp) using an electricalartifact (e.g., P-wave) as the atrial timing fiducial along with acorresponding AV interval, as seen at 712 in FIG. 14. These are justexamples. The LCP may dynamically switch between these and other atrialtiming fiducials, depending on a number of factors such as the qualityof the signals that are currently sensed. In some cases, an atrialtiming fiducial may be determined from two or more cardiac artifacts,sometimes with one weighted more than the others.

The sensing module of the LCP 100, 400 may include one or more of apressure measurement module and an acoustic measurement module. However,other measurement modules may be used as desired, including but notlimited to, measurement modules that include suitable sensors fordetermining the artifacts described with respect to FIGS. 6 and 7. Forexample, the sensing module may further include an electrogrammeasurement module. As described herein, the sensing module may beconfigured to gather information suitable for determining one or moreatrial timing fiducials. The information may include, but is not limitedto, an atrial artifact such as any of those discussed with reference toFIGS. 6 and 7. In some cases, information gathered from one of themeasurement modules may be used to determine a blanking interval foranother measurement module.

In some cases, a pressure measurement module may detect or determine atleast one of a maximum pressure (atrial or ventricular), a minimumpressure (atrial or ventricular), a mean pressure (atrial orventricular), a pressure time integral (atrial or ventricular), and/or apressure time derivative (atrial or ventricular). An acousticmeasurement module may detect or determine at least one of an S1 heartsound, an S2 heart sound, an S3 heart sound, and/or an S4 heart sound.An acceleration measurement module, if present, may detect or determineat least of an S1 heart sound, an S2 heart sound, an S3 heart sound, anS4 heart sound, myocardial (e.g., heart wall) movement, patient activityand/or patient posture. These and other artifacts may be used as thebasis for an atrial timing fiducial.

In some cases, it may be desirable for the LCP 100, 400 to be configuredto operate in a number of different pacing modes. Some illustrativepacing modes may include, but are not limited to VDD, VDDR, VVI, VVIR,VOO, and VOOR. As used herein, the pacing modes use the North AmericanSociety of Pacing and Electrophysiology (NASPE) and British Pacing andElectrophysiology Group (BPEG) pacemaker codes as outlined in Table 1below:

TABLE 1 NASPE/BPEG Revised in 2002 NBG Pacemaker Code Position IPosition II Position III Position IV Position V (Chamber Paced) (ChamberSensed) (Response to Sensed Event) (Programmability, (Multisite Pacing)O = none O = none O = none Rate Modulation) O = none A = atrium A =atrium I = inhibited A = atrium V = ventricle V = ventricle T =triggered O = none V = ventricle D = dual (A + V) D = dual (A + V) D =dual (T + I) R = rate moduation D = dual (A + V) Miller R D. Miller'sAnesthesia, 6^(th) ed. Philadelphia: Elsevier, Inc, 2005, pp 1417.A VDD device is a device pacing in the ventricle, sensing both theatrium and the ventricle, and using triggered and inhibited pacing.

It is contemplated that a right ventricle LCP 100, 400 using remotetracking of atrial activity such as described herein may automaticallyrevert from one pacing mode to another, depending on one or more sensedconditions. The reversionary behavior may be desirable for safeoperation and/or for enhancing the effectiveness of the pacing therapy.The control module may be configured to search for and identifyconditions that may indicate a reversion is desirable. Some conditionsmay include, but are not limited to: an atrial artifact (e.g., atrialtiming fiducial) occurring too close to the R-wave (or other ventricularfiducial); a hemodynamic response that indicates that the present pacingtherapy is worse than another pacing therapy or no pacing therapy,either actual or anticipated (due to one or more of posture, heart rate,respiratory rate, respiratory cycle, patient activity, physiologicalnoise, environmental noise, etc.); continuous or intermittent loss of anatrial tracking artifact or fiducial; actual or anticipated continuousor intermittent loss of an atrial tracking artifact or fiducial due tosearch algorithms associated with reacquiring an atrial artifact orfiducial; a time period between adjacent atrial artifacts or fiducialsbeing too short (e.g., due to over sensing caused by physiological orenvironmental noise or atrial tachyarrhythmia); and/or a ventricularinterval being too short (e.g., due to over sensing caused byphysiological or environmental noise or ventricular tachyarrhythmia).These are just some examples. Other events and conditions may bedetected and cause reversionary behavior.

The LCP 100, 400 may experience or be configured to use different typesof reversionary behavior based on the current conditions. In a firstexample, the control module may be configured to change pacing modes. Inthe event of a loss of an atrial timing fiducial, an atrial rate above aspecified threshold, or atrial noise above a threshold, the LCP may beconfigured to automatically switch between VDD and VVI modes. In theevent of ventricular noise being above a threshold, the LCP may beconfigured to automatically switch between VDD or VVI and VOO modes. Inthe event of a reduction in the hemodynamic signal, the LCP may beconfigured to automatically switch between VDD or VVI and OOO modes.

In an example LCP 100, 400 reverts to a VDI mode wherein the devicecontinues to search and/or measure atrial artifacts but does not use anydetected atrial artifacts to trigger ventricular pacing. If the LCPdetermines the atrial fiducial can be reliably determined the LCPreverts back to a mode that allows triggering of ventricular paces fromthe atrial fiducial (e.g. VDD mode).

In some cases, the control module may be configured to manipulate thetracking algorithm. For example, the control module may switch betweencontinuous tracking and intermittent tracking with tracking estimate andsearch. In yet another example, the atrial timing fiducial signal may bereverted. In another example, the type of signal and/or portion ofsignal used to determine the atrial timing fiducial may be changed orswitched. In yet another example, the atrial timing fiducial may bechanged from a first weighted average of two or more signals to a seconddifferent weighted average of the same or different two or more signals.These are just examples.

FIG. 15 is a flow chart 800 of an illustrative method for determiningwhether the LCP 100, 400 should utilize reversion. The control modulemay continuously verify the current pacing mode is the best under thecurrent conditions. If a reversion is needed, the control module maydynamically change the pacing mode (e.g., change on a beat-to-beatbasis, if needed). The LCP 100, 400 may first deliver a pace, as shownat 802. After delivering the pace, the control module may check to seeif the atrial artifact and/or event (e.g., atrial timing fiducial) wasdetected, as shown at 804. If the atrial timing fiducial was detected,the LCP 100, 400 may continue with its normal operational mode, which insome cases may be VDD tracking with the pacing occurring after thecorresponding AV interval, as shown at 806. An exception may occur whenthe normal VDD tracking inhibits a pacing pulse due to preventricularcontraction, as shown at 808. If the atrial timing fiducial was notdetected or resolved, the LCP 100, 400 may enter a reversionary mode, asshown at 810. In some cases, in the reversionary mode, the LCP 100, 400may enter a VDD pseudo tracking mode in which the LCP 100, 400 pacesusing an estimated atrial timing fiducial time. Other reversionary modesmay be used as appropriate. The control module may be configured toselect a ventricular pacing therapy and/or mode based at least in parton one or more of the tracked atrial artifacts/events. In some cases, afirst ventricular pacing therapy may have a first pacing rate and thereversionary (or second) ventricular pacing therapy may have a secondpacing rate different from the first. For example, the reversionaryventricular pacing therapy may extend the pacing rate to aid in thesearch for the atrial timing fiducial.

FIG. 16 illustrates a comparison of pacing intervals on anelectrocardiogram when the LCP 100, 400 is operating in a normal VDDmode 820 and pacing intervals on an electrocardiogram when the LCP 100,400 is operating in a VDD pseudo tracking mode 830. As can be seen inthe normal VDD mode 820, the control module is detecting an atrialtiming fiducial 822, and using the appropriate AV delay 824, deliversthe pacing pulse 826 at the appropriate time. The LCP 100, 400 willcontinue to operate in this manner unless conditions change that makeVDD pacing unsafe or less desirable.

FIG. 16 illustrates an example in which the control module of the LCP100, 400 has determined that the atrial artifact/event 832 is missing orunreliable. The control module may then sense a ventricular event 834(such as but not limited to the R-wave) and essentially use theventricular event 834 as the atrial pacing fiducial for the next cardiaccycle. The control module may determine an appropriate AV interval 836,calculated using the ventricular intrinsic interval (e.g., R-wave tosubsequent R-wave) minus a previously stored pace to R wave interval. Inthe example shown, starting at the sensed ventricular event 834, thepacing pulse 838 may be delivered at a time equal to the R-wave toR-wave intrinsic interval minus a percentage of the historical AVinterval. It is contemplated that the percentage of the historical AVinterval may be in the range of about 30 to 70%. Alternatively, a fixedtime period, such as, but not limited about 200 milliseconds may be usedin place of the percentage of the historical AV interval or a previouslystored pace to R wave interval. The control module may then continue topace using the R-wave to R-wave intrinsic interval 840 as the timinginterval using the pacing pulse 838 as the timing fiducial until asuitable atrial timing fiducial is identified. If the control modulefails to re-acquire a suitable atrial timing fiducial, the controlmodule may command the device to enter a search mode in an attempt todetect an intrinsic atrial and/or ventricular event. This is just oneexample of a reversionary scenario.

The control module may be configured to determine the accuracy of theatrial timing fiducial by analyzing the effectiveness of the pacingtherapy. In one example, the control module may use the upstroke (e.g.,dP/dt or peak pressure) on sequential cardiac cycles to estimate theaccuracy of the A-wave detection. If the LCP 100, 400 is pacing at anincorrect time due to an inaccurate atrial timing fiducial, passivefilling may be reduced thereby reducing the dP/dt. Similarly, thecontrol module may use an integrator to find the area under the pressurewaveform, which may represents the filling volume of the ventricle, orimpedance between the electrodes of the LCP may be used as an indicationof ventricle volume. Poor filling volume may indicate an inaccurateatrial timing fiducial. In yet another example, the control module maybe configured to search for edges using, for example a high-pass pole tocreate a differentiator to help identify a sub-par atrial timingfiducial. If the atrial timing fiducial is determined to be inaccurate,the LCP may revert to asynchronous pacing (e.g. VOO mode).Alternatively, if the patient's intrinsic heart rate is high enough(e.g. 50 BPM), the LCP may revert to no pacing (e.g. OOO mode).

In addition to a first-order differentiator, higher-orderdifferentiation could further provide better timing fiducials (e.g.,more crisp) and other measures such as verification of signal quality. Athird order time derivative of position is known as “jerk”, which is thechange in acceleration with respect to time. FIG. 17 illustrates a graph900 of an illustrative relationship of higher order differentiation of asignal 902. The signal 902 could be any suitable signal including anegram, a pressure signal, an acceleration signal, or any other suitablesignal. The change in pressure with respect to time may be consideredthe first derivative 904. The change in the first derivative 904 withrespect to time may be considered the second derivative 906. The changein second derivative with respect to time may be considered as anequivalent to jerk 908 (or the third derivative). When the signal is apressure signal, the inflections produced by an A-wave may give thirdorder blips which could be used for timing or verifying a quality of thesignal.

Those skilled in the art will recognize that the present disclosure maybe manifested in a variety of forms other than the specific examplesdescribed and contemplated herein. For instance, as described herein,various examples include one or more modules described as performingvarious functions. However, other examples may include additionalmodules that split the described functions up over more modules thanthat described herein. Additionally, other examples may consolidate thedescribed functions into fewer modules. Accordingly, departure in formand detail may be made without departing from the scope and spirit ofthe present disclosure as described in the appended claims.

1. A leadless cardiac pacemaker (LCP) configured to sense cardiacactivity and to deliver pacing therapy to a ventricle of a patient'sheart, the LCP comprising: a housing having a proximal end and a distalend; a first electrode secured relative to the housing and exposed tothe environment outside of the housing; a second electrode securedrelative to the housing and exposed to the environment outside of thehousing; a sensing module disposed within the housing, the sensingmodule configured to detect an artifact during ventricular filling andidentify an atrial event based, at least in part, on the detectedartifact; and circuitry in the housing operatively coupled to the firstelectrode and the second electrode, and also operatively coupled to thesensing module, the circuitry configured to deliver a ventricular pacingtherapy to the patient's heart via the first electrode and the secondelectrode, wherein the ventricular pacing therapy is time dependent, atleast in part, on the identified atrial event.
 2. The LCP of claim 1,wherein the sensing module is configured to detect an atrial artifactduring passive ventricular filling.
 3. The LCP of claim 2, wherein thesensing module is configured to sense at least one of a pressure,impedance, strain, sound, rotation, acceleration, voltage, and flow. 4.The LCP of claim 2, wherein the artifact comprises at least one of ap-wave, S2 heart sound, S3 heart sound, ventricular volume, ventricularwall dimension, ventricular blood movement, ventricular wall movement,tricuspid valve position, mitral valve position, and akineticventricular pressure.
 5. The LCP of claim 1, wherein the sensing moduleis configured to detect an atrial artifact during active ventricularfilling.
 6. The LCP of claim 5, wherein the sensing module is configuredto sense at least one of an impedance, strain, sound, rotation, or flow.7. The LCP of claim 5, wherein the artifact comprises at least one of ap-wave, a-wave, e-wave, S1 heart sound, S4 heart sound, ventricularvolume, ventricular wall dimension, cardiac tissue vibration,ventricular blood movement, ventricular wall movement, tricuspid valveposition, and mitral valve position.
 8. The LCP of claim 1, wherein thesensing module is configured to detect an atrial artifact by detecting atransition between passive and active ventricular filling.
 9. The LCP ofclaim 1, wherein the circuitry is further configured to determine anintrinsic interval based, at least in part, on the atrial event.
 10. Animplantable medical device configured to sense cardiac activity and todeliver pacing therapy to a patient's heart, the implantable medicaldevice comprising: a housing; a first electrode secured relative to thehousing and exposed to the environment outside of the housing; a secondelectrode secured relative to the housing and exposed to the environmentoutside of the housing; a sensing module disposed within the housing,the sensing module configured to: sense during passive ventricularfilling of the patient's heart at least one of a pressure, impedance,strain, sound, rotation, acceleration, voltage, and flow; detect anartifact; and identify an atrial event based, at least in part, on thedetected artifact; and circuitry in the housing operatively coupled tothe first electrode, the second electrode, and the sensing module, thecircuitry configured to deliver a ventricular pacing pulse to thepatient's heart via the first electrode and the second electrode,wherein a timing of the ventricular pacing pulse is dependent, at leastin part, on the timing of the identified atrial event.
 11. Theimplantable medical device of claim 10, wherein the sensing modulecomprises a filter for filtering a signal for the at least one ofpressure, impedance, strain, sound, rotation, acceleration, voltage, andflow.
 12. The implantable medical device of claim 11, wherein the filterincludes a first filter for passing a first frequency band, a secondfilter for passing a second frequency band and a third filter forpassing a third frequency band.
 13. The implantable medical device ofclaim 10, wherein the sensing module is configured to calculate anensemble average timing over multiple heart beats for the detectedartifact.
 14. The implantable medical device of claim 10, wherein thedetected artifact comprises at least one of a p-wave, S2 heart sound, S3heart sound, ventricular volume, ventricular wall dimension, ventricularblood movement, ventricular wall movement, tricuspid valve position,mitral valve position, and akinetic ventricular pressure.
 15. Theimplantable medical device of claim 10, wherein the implantable medicaldevice is configured to be implantable in a ventricle of the patient'sheart and the sensing module identifies an atrial event based, at leastin part, on the detected artifact.
 16. An implantable medical deviceconfigured to sense cardiac activity and to deliver pacing therapy to apatient's heart, the implantable medical device comprising: a housing; afirst electrode secured relative to the housing and exposed to theenvironment outside of the housing; a second electrode secured relativeto the housing and exposed to the environment outside of the housing; asensing module disposed within the housing, the sensing moduleconfigured to: sense during active ventricular filling of the patient'sheart at least one of a pressure, impedance, strain, sound, rotation,acceleration, voltage, and flow; detect an artifact; and identify anatrial event based, at least in part, on the detected artifact; andcircuitry in the housing operatively coupled to the first electrode, thesecond electrode, and the sensing module, the circuitry is configured todeliver a ventricular pacing therapy to the patient's heart via thefirst electrode and the second electrode, wherein the ventricular pacingtherapy is time dependent, at least in part, on the identified atrialevent.
 17. The implantable medical device of claim 16, wherein thesensing module comprises a bandpass filter of about 1 Hertz (Hz) toabout 5 Hz.
 18. The implantable medical device of claim 16, wherein thesensing module comprises a bandpass filter of about 15 Hertz (Hz) toabout 30 Hz.
 19. The implantable medical device of claim 16, wherein theartifact comprises at least one of a p-wave, a-wave, e-wave, S1 heartsound, S4 heart sound, ventricular volume, ventricular wall dimension,cardiac tissue vibration, ventricular blood movement, ventricular wallmovement, tricuspid valve position, and mitral valve position.
 20. Theimplantable medical device of claim 16, wherein the implantable medicaldevice is configured to be implantable in a ventricle of the patient'sheart and the sensing module identifies an atrial event based, at leastin part, on the detected artifact.